The present invention relates to color graphics image processing. It finds particular application in conjunction with detection and segmentation of sweeps in color graphics images, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
Content-based image classification has emerged as an important area in multimedia computing due to the rapid development of digital imaging, storage, and networking technologies. A reproduction system, such as a copier or a printer, strives for a pleasing rendering of color documents. Picture/graphics classifiers have been developed to differentiate between a picture image and a graphics image with high accuracy by analyzing low-level image statistics.
For example, U.S. Pat. No. 5,767,978 to Revankar et al. discloses an adaptable image segmentation system for differentially rendering black and white and/or color images using a plurality of imaging techniques. An image is segmented according to classes of regions that may be rendered according to the same imaging techniques. Image regions may be rendered according to a three-class system (such as traditional text, graphic, and picture systems), or according to more than three image classes. In addition, only two image classes may be required to render high quality draft or final output images. The image characteristics that may be rendered differently from class to class may include half toning, colorization and other image attributes.
A color output device such as a CRT computer monitor, liquid crystal display, inkjet printer, xerographic printer, etc can display a limited range of colors (the gamut of the output device). If the colors in an image do not reside wholly with in an output device gamut, a gamut-mapping algorithm is often applied to map the image colors to colors that the output device can produce. A simple approach is to preserve in-gamut colors without alteration and clip out-of-gamut colors to the closest in-gamut color. More sophisticated techniques can be used. Ideally, the gamut-mapping algorithm used should be tailored to the image type. For example, a region of smoothly varying colors should appear smoothly varying on the output device. Were the colors of a sweep to exceed the gamut of an output device, the aforementioned clipping approach will show disagreeable artifacts. In fact, it may be desirable to sacrifice color fidelity within the gamut to achieve a smooth color transition. Thus knowing that a region is, or contains, a sweep aids in color reproduction. In general, coloring schemes (gamut-mapping algorithms) are tailored for specific types of images to obtain quality reproduction. Once an image has been identified as a graphics image, further identification of image characteristics can be used to fine-tune the coloring schemes for more appealing reproductions. The most prominent characteristics of a graphics image include patches or areas of the image with uniform color and areas with uniformly changing colors. This invention focuses on the identification of the second characteristic.
One example where areas with uniformly changing color can usually be observed is in the gradient backgrounds of color business presentation slides. These areas of uniformly changing color are called sweeps and are constructed in the three-dimensional color space as a line during the construction of the synthetic graphics. A sweep is constructed by a mathematical formula to cause adjacent pixels to change color in a smooth, predictable way. For example, one can use linear interpolation of two colors specified for the sweep and render the original image by plotting pixels of interpolated colors such that neighboring spatial regions are rendered with colors from neighboring color regions. One can contemplate other mathematical descriptions of curves that achieve like effects. If such a document is printed or scanned, the sweeps do not exactly contain the colors on the line due to halftone noise introduced. If a reproduction system can correctly identify and segment the sweep areas in an image, the original sweeps can be reconstructed in the color space and rendered. The sweeps thus rendered will be very smooth and the noise introduced by the halftone will not be reproduced. Secondly, if the extreme colors of the sweep can be automatically identified, the coloring schemes can be tailored to maximize the smoothness as well as contrast and differentiation among colors to render business graphics documents.
Further identification of the properties of the graphics image can be used to fine-tune the coloring scheme to obtain a more appealing reproduction. The detection of sweeps in a graphics image can be used to reconstruct synthetic sweeps that may otherwise be perturbed due to half toning, scanning artifacts, or aging of a document or for other reasons. The extent of the sweeps (i.e., the change from color 1 to color 2) may also be used to tailor the coloring scheme to achieve best smoothness, contrast and differentiation among colors in the reconstructed sweeps.
The present invention proposes a new and improved method for detecting and segmenting sweeps in a color graphics image that overcomes the above-referenced problems and others.